Preheat oven to 350° F. Line a square baking pan with parchment paper, allowing some overhang.
Heat sweetened condensed milk and butter together in a medium saucepan over medium heat, stirring frequently. Once melted, remove from heat and add vanilla extract and brown sugar. Stir to incorporate.
Mix in cocoa powder, salt, baking powder and flour and mix until thoroughly blended. Pour batter into prepared baking pan.
Bake for 30 minutes or until middle is just set and sides are slightly crisped.
Rest in pan for 5 minutes before using overhang of parchment paper to pull brownies from baking pan. To serve, cut into squares and enjoy warm. Cool completely before storing in an airtight container.